We used Hotwire before. Actuall on a trip to Vegas from Wichita. Total price was $202, which is about the lowest it gets with sales. We used Hotwire because no one was having a sale that cheap.
Flight times were leaving Wichita at 7AM and leaving Vegas at 7AM. Everytime we've used either Priceline of Hotwire for Vegas, departures from Vegas have been 7AM.
I usually check Hotwire first, then place a bid on Priceline lower than the Hotwire quote. To date, there's only been one time that Priceline hasn't beat Hotwire.
Beware you won't always get a good deal on either site. Hotwire is often as high as, or even higher than what I can book without it. Sometimes Priceline rejects my bid and comes back with an offer price as high, or higher, than what I can book it without using them.